Within a duct traveled by a fluid, there is a velocity gradient both in the horizontal direction and the vertical direction of the wet straight section, the velocity being theoretically close to zero at the walls of the duct. It is therefore necessary to determine a mean velocity of the fluid. However, flowmeters generally take a occasional or partial measurement of velocities at the surface of the liquid or in the wet straight section. Thus, technologies based on velocity measurements such as measurements by propellers, turbines, force, Vortex, thermal measurements, measurements by transit-time ultrasounds, Doppler-effect ultrasounds, cross-correlation ultrasounds and, more particularly, surface velocity measurements in open channels by radar microwave, acoustic waves, optics and lasers all encounter the same problem of converting a precise local measurement of velocity into a correct mean velocity of the wet section, this mean velocity being next multiplied by the surface of the wet section to obtain the flow rate.
The most common manner for resolving this conversion problem is to calibrate the flowmeter on a calibration bench. This calibration technique is very often used for pressure ducts. For open channels, it is difficult or even impossible to reproduce all of the elements of the measuring site in a hydraulic laboratory or on a calibration bench. In order to refine the conversion of the local or even occasional velocity measurement in mean velocity measurement of the wet section, several calibration techniques are used. The most common is to read a matrix of velocity-measuring points by means of current meters, electromagnetic velocity probes, Doppler probe, laser probe or any other occasional velocity measuring device and convert that reading of velocity measurements into a mean velocity for the water level and the hydraulic conditions encountered during the reading. The calibration can also be done using reference flow rate measurements. These calibrations must be repeated for different water levels that may occur in the open channel or partially filled duct.
The hydraulic conditions are generally defined and characterized by the water level only, but other parameters (temperature, pH, conductivity, turbidity, etc.) can also be taken into consideration to define particular hydraulic flow conditions. In most cases, the level measurement is the element taken into account to characterize the hydraulic flow conditions. In some particular cases, when the level varies little, the occasional or local velocity measurement can be taken into account to characterize the hydraulic flow conditions. In more particular situations where there is a blockage of the flow downstream, a combination of the occasional or local velocity measurement and the level make it possible to determine the hydraulic conditions. Once the hydraulic conditions are defined by a measurement or set of measurements, a number of correction vectors corresponding to each hydraulic condition may be read by calibration on the measuring site, and converted into a conversion table (generally a conversion table depending on the level with linear interpolation between the different measuring points) or a correction model.
Apart from the difficulty to perform this reading precisely, this method has the tremendous drawback to require a lot of time to perform the readings under the different hydraulic flow conditions. It will also be noted that it is sometimes impossible to perform these readings for technical or operator's safety reasons.
In order to avoid these drawbacks, new technologies have been patented or applied.
U.S. Pat. No. 7,672,797 B2 describes a method consisting in determining the maximum velocity on a vertical velocity profile and in applying a multiplier factor on this maximum velocity to compute the mean velocity.
U.S. Pat. No. 5,811,688 describes a method based on a local velocity measurement at the surface of the fluid consisting in applying multiplier factors on the measured surface velocity, the multiplier factors depending on the level of fluid in the duct.
Other technologies based on mathematical models can also be used. One of the best known is the finite element model by Dr. Kölling, as described in patent EP 0 681 683 B1. It simulates a set of flow-velocity distributions in a channel having a known profile, for several liquid levels in the channel. Based on a measurement of the liquid level and a velocity measurement, it next selects the simulated velocity distribution that is appropriate for the measured liquid level.
The main drawback of mathematical models is that they must be supplied with data characterizing the channel or the duct. This data includes the slope and roughness, which are difficult to determine precisely; the roughness may also evolve over time.